The iBiquity Digital Corporation HD Radio™ system is designed to permit a smooth evolution from current analog amplitude modulation (AM) and frequency modulation (FM) radio to a fully digital in-band on-channel (IBOC) system. This system delivers digital audio and data services to mobile, portable, and fixed receivers from terrestrial transmitters in the existing medium frequency (MF) and very high frequency (VHF) radio bands.
IBOC signals can be transmitted in a hybrid format including an analog modulated carrier in combination with a plurality of digitally modulated subcarriers, or in an all-digital format wherein the analog modulated carrier is not used. Using the hybrid format, broadcasters may continue to transmit analog AM and FM simultaneously with higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog to digital radio while maintaining their current frequency allocations. IBOC hybrid and all-digital waveforms are described in U.S. Pat. No. 7,933,368, which is hereby incorporated by reference.
Signals from adjacent radio channels can interfere with the digitally modulated carriers of a hybrid IBOC signal. A First Adjacent Canceller (FAC) technique can be used to mitigate the effects of first-adjacent FM interference to the digital sidebands of an HD Radio signal in the FM broadcast band.